1. Field of the Invention
This invention relates to a brushless DC motor, more specifically, to a brushless DC motor for a disc drive with higher fidelity which is accomplished by retaining a rotor unmovably in a axial direction using a core insulator and by centering a disc automatically when it is put on the upper end of the rotor.
2. Description of the prior art
Generally, most bearings used in a spindle motor for a digital video disc (DVD) drive or a compact disc (CD) drive are ball bearings or sintered bearings. For these motors, there should be an axially retaining structure which stops the rotor from releasing and maintains the assembled structure when a disc is inserted or removed. It consists of ball bearings and springs to control the pre-pressure of ball bearings in the case of ball bearings while the retaining of sintered fluid bearings is possible through a design of the outer structure of the rotor and the base. However, for a high speed spindle motor of a DVD drive, ball bearings are not proper due to vibration and noise, and the retaining means of the outer structure of the rotor and base has the problem that the power for rotating the rotor is relatively greater.
FIG. 1 is a sectional view of a spindle motor of the prior art using a dynamic pressure bearing. A bearing1a is forcibly inserted into a bore formed in an upper and central portion of the base. A shaft 2 is inserted into the bearing1a. A stator1b is provided on the outer circumferential face of the base 1 and a cap-type rotor 3 is secured to the upper end of the shaft 2. A magnet3a is provided on an inner circumferential face of the rotor. When power is applied to the coils wound in the stator a magnetic force arises in the magnet and the rotor rotates. On the upper end of the rotor, a disc 4 is located and a disc clamp 5 is provided at the bottom of a door 6.
The conventional spindle motor has a groove 2a for generating a dynamic pressure. When the shaft 2 rotates at high speed, the oil filled between the shaft 2 and the bearing 1a generates dynamic pressure and the shaft 2 is retained in the radial direction by the pressure. While the shaft rotates, a disc located on the upper surface of the rotor rotates and information written in the disc is read. However, the rotor 3 released, or could be moved in the axial direction of the shaft. The shaft is not retained in the axial direction due to the minute space between the shaft 2 and the bearing 1a for generating the dynamic pressure. Therefore, when the grip of the disc clamp 5 is released the rotor 3 could be lifted by magnets formed at the bottom face of the disc clamp 5. To solve the problem a structure between the rotor 3 and the base has been suggested but has failed to meet the requirements of low vibration and noise due to relatively greater unbalance of the rotating rotor. In addition, there has been no desirable means of adjusting the center of rotation of a disc to alleviate unbalance. Insertion of the disc through the hole of the disc into a projection at the upper end of the rotor 3 impairs the loadability of the disc.